This invention relates generally to memory retention systems for electronic devices having volatile memory, and particularly to such systems for electronic equipment having at least two logic devices clocked by a common oscillator.
Many types of electronic equipment include logic devices, such as microprocessors, microcomputers and custom logic integrated circuits, which are used to store data and to perform various control functions of the equipment. Frequently, in an effort to minimize the production cost, the logic devices are clocked by a common oscillator. The storage mechanism of many logic devices is volatile memory. As is known to those skilled in the art, when an electronic apparatus which includes volatile memory logic devices is de-energized the data stored in the logic devices are completely lost. For this reason, it is preferable that such equipment include a means for maintaining a biasing voltage on the logic devices so that a failure of, or a significant decrease in, the biasing voltages does not result in a rapid memory loss by the various logic devices.
An example of equipment which includes several logic devices, at least one of which has a volatile memory, is a television receiver which can be remotely controlled by a handheld, infrared (IR) remote control unit. Such television receivers typically include custom logic integrated circuitry (IC) which decodes and holds the commands received from the remote control unit. Such receivers also typically include a microprocessor or microcomputer, typically a random access memory (RAM), to store data received from the custom logic IC and also to make any data manipulations needed by the custom IC to effect the various operating functions of the receiver. Typically, at least one, and frequently both, of the logic devices includes a volatile memory and therefore are subject to memory loss in the event of the failure of the biasing voltages to the logic devices. It should be understood that the loss of the biasing voltages to the logic devices does not occur when the receiver is turned off. The biasing voltage losses occur when the television receiver is unplugged from the AC voltage source, or in the event of a failure of the AC supply lines.
The custom logic integrated circuit includes an eight bit register which stores the control information received from the remote control unit. For example, assume that the television viewer wishes to change the volume of the receiver. The viewer pushes the volume key on the handheld remote control unit. This results in the IC receiving a data stream which is indicative of the desired volume change. The IC decodes and stores the desired command as an eight bit value. The microprocessor periodically queries the IC to determine whether or not a command has been received from the hand held unit. The desired volume change is noted by the microprocessor which makes any data manipulations needed to effect the change and inputs the newly manipulated data into the custom IC to effect the change. The microprocessor also stores the data which were input to the analog interface unit AIU. Any delay between the reception of the change from the remote control unit and the execution of the command is in the order of a few milliseconds and the operator does not notice any undesirable delay.
When a receiver of the type mentioned above is initially installed in the viewer's home, the receiver is energized by the standard 110V AC power available in the viewer's home. The viewer is provided with an instruction book which contains instructions on how to program the tuner to scan the local channels or those which are available on cable. The viewer is also instructed on how to program other functions, such as setting a clock which is included in the receiver, into the IC memory. When the logic devices are volatile memory devices, a momentary loss of power to the receiver results in the loss of all the stored data and the viewer must completely reprogram the receiver. In the prior art, memory retention has been accomplished by placing a capacitor at the biasing terminal of the logic device to retain a voltage on the memory for a period of time following power loss.
The logic devices require a clocking oscillator and in order to minimize the manufacturing cost of the receiver, it is a common practice to utilize the same oscillator for all the logic devices within the receiver. However, during power loss the oscillator will be energized by a voltage developed across the capacitor intended to retain the biasing voltage on the memory, thereby quickly depleting the charge on the capacitor and substantially decreasing the time available for memory retention. For these reasons there is a need for a system for preventing the loss of the data stored in volatile memory logic devices within electronic equipment during periods in which the supply voltage drops below an acceptable level, and also for turning off the clocking oscillator during such low voltage periods. The present invention fulfills these needs.